Hi,
The places I've found them are:
Titirangi Beach, the stream just before the bridge to the kids playground.
Mill Bay (Cornwallis), the streams just before the bridge.
All you need is a net and to jump into the streams where the grass hangs into the water. Sweep the net just under the overhanging grass and you should get some.
Mine just eat the left over flake, blood worms or tubifex worms that I feed other fish.